Electronic fingerprint sensing has received increased attention as a technique for reliable identification of individuals. Electronic fingerprint sensing may be used in stationary equipment, such as security checkpoints, and in portable devices, such as mobile phones and other wireless devices, and smart cards. Accordingly, electronic fingerprint sensing systems are required to be compact, highly reliable and low in cost.
Various electronic fingerprint sensing methods have been proposed. Known methods include optical sensing and capacitive sensing with a two-dimensional array of electrodes.
Capacitive fingerprint sensing using a swiped finger technique is disclosed in International Publication No. WO 02/47018, published Jun. 13, 2002. Conductive elements, or plates, are formed on an insulating substrate to create a one-dimensional capacitive sensing array for detecting topographic variations in an object, such as a finger. The array includes multiple drive plates which are sequentially excited with short duration electronic waveform bursts. An orthogonal pickup plate spaced from each drive plate by a sensor gap and connected to a charge sensing circuit detects the intensity of the electric field created by each drive element. With each complete scan of the drive plates, a one-dimensional slice of the fingerprint is acquired. By swiping a finger across the gap between the drive plates and the pickup plate, and scanning the gap at a much faster rate than the swipe speed, a two-dimensional image based on capacitance is generated. The image represents the fingerprint.
Fingerprint sensors of this type provide satisfactory performance but are subject to parasitic coupling and noise combined with interference coupled through the body of the finger from finger ridges outside the sensor gap. Accordingly, there is a need for improved electronic fingerprint sensing apparatus and methods wherein the above effects are reduced.